Mono C1–C4 alkyl phosphoric acid esters have been used for many years as wetting agents, detergents and aqueous corrosion inhibitors; however the short chain alkyl group is only capable of delivering a very limited number of benefits. Furthermore, these short chain alkyl phosphate esters, albeit water soluble, tend to realize unacceptable foaming under certain conditions.
To obtain additional benefits from the alkyl moiety and minimize the foaming characteristics, the art extended the length of the alkyl chain to the range of from about C6–C30. This long chain alkyl moiety provided additional properties that enabled the oil soluble monoalkyl phosphate esters to be used in secondary oil well recovery systems; as asphalt additives; and as components in power transmission fluids.
However, the benefits realized by the alkyl moiety also limited the uses of these phosphate compounds because the hydrophobicity of the alkyl chain impaired the use of the compounds in aqueous formulations. As a result, the art explored a third approach which was to replace the mono alkyl long chain radical in the phosphate compounds with a polyoxyalkylene moiety.
Polyoxyalkylenes per se, such as homopolymers and copolymers of alkylene oxide (EO), propylene oxide (PO), and butylene oxide (BO) are relatively inexpensive and are a very useful class of polymers, i.e., they have a very broad applications spectrum. They function as plasticizers in the adhesives, ceramics, and paper industries; as lubricants in the synthetic lubricant, metal working, natural and synthetic rubber, and textile industries; as protective coatings in the electroplating, electropolishing, and plastics molding industries; and as foam control agents in water and wastewater treatment facilities.
Replacing the mono long chain alkyl group on the phosphoric acid with a polyoxyalkylene homopolymer or copolymer moiety was fairly successful in achieving water miscibility together with a reduction in foaming characteristics. These mono alkoxylated phosphate esters have been disclosed as being used as dispersants for organic and inorganic particles in aqueous compositions, for example, as compatibility surfactants to reduce clustering of pigments in latex paints. They have also been disclosed as being useful as thickening agents in aqueous hydraulic fluids; as corrosion inhibitors; and as scale inhibitors in aqueous coolant compositions.
However, although the polyoxyalkylene phosphate esters could theoretically provide many additional useful benefits, they suffer from some serious drawbacks vis-a-vis their commercial desirability. At the outset, mono alkyl and mono polyoxyalkylene phosphate esters, albeit highly desirable because of the substrate adhesion force realized by the phosphate moiety, are extremely difficult to prepare in high yields and purity. Current syntheses tend to produce significant quantities of the less desirable dialkyl phosphates, dipolyoxyalkylene phosphates, and impurities that require extensive and expensive purification and isolation procedures. For instance, conventional processes give a mixture of mono alkyl and dialkyl phosphate esters with a ratio of no higher than 90:10. Efforts to suppress the formation of the dialkyl esters tend to result in the production of a large amount of corrosive phosphoric acid in the product composition.
Furthermore, while the phosphate moiety will bind or anchor to many different substrate surfaces, it will do so only under fairly mild conditions. That is, the phosphates tend to be both thermally and hydrolytically unstable. At elevated temperatures, the phosphates are susceptible to hydrolysis under both basic and acidic conditions.
It has been recognized in the art that long chain alkyl phosphonates having the general structure of formula I:

wherein R1 is independently H or C1 to C4 alkyl and R2 is C6 or higher alkyl, are much more stable than the phosphates, both thermally and hydrolytically, and they are also known to react or interact with surfaces of many different materials. They have a very strong affinity for metal, particularly aluminum, and inorganic salt particles. These phosphonates are used to prevent the staining of aluminum surfaces due to oxidation and they also provide a lubricity effect to the metal, i.e., coatings utilizing these long chain phosphonates on metal substrates are able to decrease contact wear during processing or use of the metal.
The long chain alkyl phosphonates are also used as coatings on inorganic fillers or metal pigments to improve their dispersions by lowering the energy required to disperse the filler or pigment, especially in polymeric substrates. Use of these dispersant alkyl phosphonates increases the amount of filler that can be incorporated into a master batch thereby improving the storage stability of the filler; the processing of the filler into the plastic matrix; and the physical properties of the filled polymer product.
The long chain alkyl phosphonates have also been used in organic environments as corrosion inhibitors, plasticizers, synthetic lubricants, flame retardant additives, and textile treatment agents. As a result of their high oil solubility, they are used in power transmission fluids, extreme pressure lubricant additives, and as high melting grease thickening agents.
In both the mono alkyl phosphate and phosphonate cases, the phosphate or phosphonate group serves as the anchor, i.e., provides the adhesive force to bind the compound to the substrate and the mono alkyl chain on the C—O—P or C—P bond, as the case may be, provides the benefits; whether it is a lubricity, or dispersion, or an anti-staining/anti-oxidative property. In the case of these long chain alkyl molecules, the achievement of these benefits can be attributed to the hydrophobization of the surface of the substrate by the group attached to the carbon atom of the C—O—P or C—P bond.
However, as with the phosphate compounds discussed above, one of the drawbacks of the alkyl phosphonates is that they have a very narrow range of applications due to the fact that the alkyl group is (i) only capable of delivering a very limited number of benefits; (ii) the C1–C4 alkyls tend to foam under certain conditions; and (iii) the long alkyl chain phosphonates are not water soluble which, of course, hinders their applications in aqueous formulations.
The very diversity of these various phosphate and phosphonate compounds is an indication that all are not equally useful. It is not unusual to find some of these compounds function quite well, for example, providing a dispersing or coating property in one environment and not at all in another environment. Thus, there is a continual need and a continuing search for new phosphorous containing compounds and compositions, such as the phosphonates, and methods for preparing same to satisfy many unmet needs.
Accordingly, it is an object of this invention to provide phosphonate compounds and compositions with a range of applicability and usefulness equal to or exceeding many of polyoxyalkylene homo- and co-polymers and having, in addition, enhanced substrate adhesion properties.
It is also an object of this invention to provide a straightforward, relatively inexpensive process for the production of these improved phosphonates in high yields and purity.
Additionally, it is a further object of this invention to provide a process for the synthesis of these phosphonate compounds wherein the moiety attached to the carbon atom on the C—P bond of the phosphonate is optionally end-capped with a hydroxyl group.
Other objects will be evident from the ensuing description and appended claims.